The overarching hypothesis of the proposed research is that loss of function of the Pin1 prolyl isomerase can result in differential susceptibility to tumorigenesis. Pin1 was originally identified as a regulator of a fungal protein kinase required for mitosis and so was suggested to play important roles in cell cycle progression. It is now known that Pin1 functions in both G1 and G2 of the cell cycle and several Pin1 binding proteins that also play important roles in the cell cycle have been identified. Pin1 binds to proteins phosphorylated on pSer/pThr-Pro motifs and isomerization of the prolyl amide bond can cause a conformational change that regulates the abundance of the target protein in the cell either in a positive or negative manner. Pin1 has been implicated to be important in human cancers not only because it is involved in cell cycle regulation but also as several studies have reported either increased levels of Pin1 in human tumor specimens or Loss of Heterozygosity of the portion of chromosome 19p13.3 that contains Pin1. Thus, the question of whether the loss of Pin1 could, depending on the genetic background, either sensitize or protect cells from transformation remains enigmatic. Evaluation of fibroblasts from Pin1 null mice in a pure C57BL6/J genetic background revealed two novel target proteins for Pin1 to be c-Myc, and cyclin E1. Pin1 regulates the degradation of c- Myc. The absence of Pin1 from immortalized mouse fibroblasts increases c-Myc and cyclin E1 levels, results in genomic instability and renders these cells sensitive to Ras-mediated transformation. To extend these remarkably provocative results and address this fascinating conundrum, it is proposed to: 1) evaluate the mechanism of action of Pin1 by using NMR to determine how Pin1 binds to c-Myc and biochemistry/cell biology to evaluate whether Pin1 regulates accumulation of cyclin E1 as it does c-Myc; 2) examine how the loss of function of Pin1 contributes to genomic instability in immortalized mouse fibroblasts; and 3) test the hypothesis that loss of function of Pint can alter the timing of tumorigenesis in a genetically engineered mouse model of cancer wherein spontaneous recombination in whole animals is necessary to activate an oncogenic allele of K-Ras, which in turn leads to tumorigenesis in 100% of the mice by 300 days. We hope that completion of these proposed studies will help to clarify the potential role of Pin1 and/or its target proteins in carcinogenesis.